The present invention relates generally to satellite positioning systems and, in particular, to a telecommunications-based auxiliary system that assists a wireless terminal in determining its position from signals transmitted from one or more navigation satellites.
FIG. 1 depicts a prior art satellite positioning system 100, also referred to herein as a Wireless Assisted Global Positioning System (WAG). The satellite positioning system depicted comprises wireless terminal or WAG client 501, satellite constellation 503, auxiliary system or WAG server 505, and timing source 507. Satellite constellation 503 is the Global Positioning System (GPS) comprising a plurality of satellites for transmitting GPS signals that convey a variety of location related information including ranging information and satellite ephemeris information. GPS is well-known in the art. It will be clear to those skilled in the art how to make and use embodiments of the present invention that work with other satellite constellations. WAG server 505 and WAG client 501 are operable to acquire GPS signals and process GPS signals (i.e., demodulate and extract) in order to obtain the information being conveyed by the GPS signals.
The principal goal of WAG is to reduce the signal acquisition and signal processing requirements of a conventional wireless terminal so that a wireless terminal in accordance with the illustrative embodiment can determine its location more quickly and with weaker signals than wireless terminals in the prior art. In accordance with WAG, the signal acquisition and signal processing requirements of wireless terminal 501 are reduced at the expense of auxiliary system 505. In particular, the tasks of signal acquisition and signal processing required for a conventional wireless terminal to determine its position are divided between wireless terminal 501 and auxiliary system 505.
It will be clear to those skilled in the art how the signal processing task can be divided between wireless, terminal 501 and auxiliary system 505, as partially processed signal information can be exchanged back and forth between the two through wireless telecommunications link 504 as needed to achieve desirable division of the signal processing task.
It is possible to divide the signal processing task between wireless terminal 501 and auxiliary system 505 because each signal transmitted by each satellite in satellite constellation 503 conveys ranging and satellite ephemeris information that are responsive to independent acquisition and independent processing. Specifically, a GPS signal transmitted from a particular satellite will convey the same satellite ephemeris information to a receiver (e.g., WAG client or WAG server) regardless of which receiver acquires and processes the signal. Basically, the GPS signal is modulated with digital information in a manner similar to how, for example, a cellular telephone""s radio signal is modulated with voice data. Such information can be detected and demodulated by any receiver adapted to do so. The information reconstructed by the receiver is an exact replica of the information modulated onto the signal by the transmitter (except for unwanted errors due to noise, distortion, etc.) and is the same for all receivers, regardless of their position.
By contrast, a GPS signal transmitted from a particular satellite will convey different ranging information to different receivers acquiring and processing the GPS signal. The transmitter (i.e., satellite) carefully adjusts the timing of the transmitted signal according to some precise reference, such that the timing of the signal, as received by the receiver, carries information about the distance between the transmitter and the receiver (and, therefore, about the receiver""s position). Such information will be different from receiver to receiver, and is only available at the receiver itself.
For example, since each satellite in constellation 503 transmits a signal 502 that contains both kinds of information to both wireless terminal 501 and auxiliary system 505, some or all of the information about the satellites"" ephemeris is acquired by auxiliary system 505 through antenna 553, even though the ranging information acquired by auxiliary system 505 is relevant to the position of auxiliary system antenna 553 and not to the position of wireless terminal 501. However, auxiliary system 505 has approximate knowledge of the position of wireless terminal 501 (for example, through knowledge of the cell and sector where the wireless terminal is located); therefore, auxiliary system 505 combines this knowledge with the acquired ranging and satellite ephemeris information to compute an estimate of the ranging information at the position of wireless terminal 501. This estimate, together with the satellites"" ephemeris information, is transmitted, via wireless telecommunications antenna 551, to wireless terminal 501 to assist wireless terminal 501 in acquiring and processing ranging information.
Once the ranging information has been acquired by wireless terminal 501, wireless terminal 501 can use the satellite ephemeris information and ranging information to determine its location, or wireless terminal 501 can transmit the ranging information back to auxiliary system 505 so that auxiliary system 505 can determine the location of wireless terminal 501.
Because wireless terminal 501 is freed from the task of acquiring some or all of the information about the satellites"" ephemeris and is advantageously provided with an estimate of the ranging information, it can be fabricated from less expensive technology that need only perform the easier task of acquiring and processing the ranging information with a priori knowledge of an estimated form of that information. Furthermore, because the satellite ephemeris information is modulated onto the same carrier as the ranging information, the provision of the satellites"" ephemeris information to wireless terminal 501 enables wireless terminal 501 to remove the satellites"" ephemeris information from the satellite signal received through antenna 512 and, thereby, acquire the ranging information even under faded conditions of low signal-tonoise noise ratio that are inadequate for the operation of a wireless terminal in prior art.
FIG. 2 depicts a block diagram of the salient components of auxiliary system 505, which comprises: timing signal receiver 603, timing signal antenna 552, coarse location estimator 601, telecommunications system manager 617, GPS receiver 605, GPS receiver antenna 553, timing signal calibrator 607, PRN synchronization estimator 609, demodulator 611, satellite visibility estimator 613, satellite Doppler estimator 615, telecommunications transmitter 619 and telecommunications antenna 551.
In general, auxiliary system 505 uses its GPS receiver to obtain from each satellite above the horizon both ranging information and information about the satellite""s ephemeris, in well-known fashion using a Coarse Acquisition (C/A) code. It will be clear to those skilled in the art how to make and use embodiments of the present invention that use a P(Y) or P code. In the process of obtaining the ranging and satellite ephemeris information, auxiliary system 505 learns, among other things: (1) the PRN synchronization from each satellite (i.e., the exact timing of the PRN code transmitted by each satellite), (2) the Doppler shift associated with each satellite, (3) which satellites are above the horizon, and (4) the 50 bps modulated bit stream from each satellite. Auxiliary system 505 then transmits to wireless terminal 501, via a wireless telecommunications channel, for each satellite above the horizon: (1) an estimate of the PRN synchronization, (2) an estimate of the Doppler shift, and (3) the 50 bps modulated bit stream. Collectively, this information will be called xe2x80x9cNavigation Message Data.xe2x80x9d
When auxiliary system 505 is part of a wireless telecommunications system that partitions a geographic area into a number of tessellated areas called xe2x80x9ccells,xe2x80x9d auxiliary system 505 knows which cell wireless terminal 501 is in and, therefore, its rough location to within a few miles. When auxiliary system 505 has a rough idea (e.g., within a few miles) of the position of wireless terminal 501, auxiliary system 505 can accurately estimate the PRN synchronization and Doppler shift as seen by wireless terminal 501.
FIG. 3 depicts a block diagram of the major components of wireless terminal 501, which comprises: terminal controller 710, user interface 720, telecommunications transmitter 741, telecommunications receiver 751, field receiver 753, timing receiver 755, duplexor 733 and antenna 731, interconnected as shown.
Advantageously, but not necessarily, wireless terminal 501 is capable of performing all of the functionality associated with a typical wireless terminal (e.g., a cellular telephone,). In particular a user of wireless terminal is advantageously capable of having a two-way voice conversation through telecommunications transmitter 741, telecommunications receiver 751 and auxiliary system 505.
Because the Navigation Message Data is transmitted to wireless terminal 501 from auxiliary system 505, the Navigation Message Data is received by wireless terminal 501 via telecommunications receiver 751. Telecommunications receiver 751 passes the Navigation Message Data to terminal controller 710, which, in turn, passes the Navigation Message Data to field receiver 753.
As discussed above, wireless terminal 501 also advantageously receives system timing for synchronization purposes. When the timing signal is transmitted from timing source 507, the timing signal is received by wireless terminal 501 via timing receiver 755. Timing receiver 755 passes the timing signal to terminal controller 710 which, in turn, passes the timing signal to field receiver 753. Alternatively, when the timing signal is transmitted from auxiliary system 505, (as is the case when wireless terminal 501 and auxiliary system 505 are part of a CDMA telecommunications system) the timing signal is received by telecommunications receiver 741. Telecommunications receiver 741 then passes the timing signal to terminal controller 710 which, in turn, passes the timing signal to field receiver 753.
In either case, field receiver 753 receives the timing information that it needs without needing to derive it from satellite constellation 503. Furthermore, field receiver 753 also receives for each satellite above the horizon: (1) an estimate of the PRN synchronization, (2) an estimate of the Doppler shift, and (3) the 50 bps modulated bit stream, again without having received any of this information directly from satellite constellation 503.
Wireless terminal or WAG client 501 uses the received information about each satellite (in the Navigational Message Data) to acquire the GPS signals transmitted by these satellites. For example, WAG client searches for GPS signals at the Doppler shift estimate and at the PRN synchronization estimate or a progressive search of synchronization positions near the PRN synchronization estimate.
The present invention is directed towards a method for estimating a PRN synchronization and Doppler shift of a satellite signal at a reference location within a cell and sector of where a wireless terminal or WAG client is located. The estimates are based on information conveyed by a GPS signal acquired by an auxiliary system or WAG server, which may be at a location different from the wireless terminal or WAG client. The PRN synchronization and Doppler shift estimates are also based on some reference time, which is a time that is sufficiently far into the future to allow the WAG client to demodulate and decode the PRN synchronization and Doppler shift estimates from an assisting message transmitted by the WAG server and to use such estimates to acquire GPS signals, wherein the assisting message indicates the PRN synchronization and Doppler shift estimates.
In one embodiment, the assisting message can be broadcast to WAG clients. The assisting messages can be used by WAG clients to locate themselves; no additional help by the WAG server is required and no reverse-link communication is needed. This has the added advantage of privacy, as the system has no knowledge of which WAG client, if any, are locating themselves. In another embodiment, the assisting message is transmitted to WAG clients on demand. In this embodiment, the WAG client has established a communication link, wants to be located immediately and is aware that a location operation is going on. In this mode, the terminal requests assistance from the WAG server and, after an exchange of a few messages, its location is determined with help from the WAG server.